1. Field of the Invention
The present invention relates to a touch panel circuit, and more particularly to a noise-elimination circuit of a touch panel.
2. Background of the Invention
In portable terminals using pen input on a touch panel in the past, an LCD (liquid crystal display) was used as the means for displaying information, and a touch panel was used as a means for inputting the required, characters, images, and other information.
A touch panel is usually laid over the LCD, with information often input in relationship to an image that is displayed on the LCD via an interface.
In a portable terminal, because of the tradeoff between cost and power consumption, a reflective type of STN (super twisted nematic) LCD is often used.
Because of the nature of this STN-type LCD, if the voltage polarity is fixed, the liquid crystal element exhibits deterioration and, for this reason, in actual use alternating current is applied to the LCD to inhibit such deterioration.
To achieve this, a signal that is made an AC signal is applied to the liquid crystal, so that the alignment thereof is changed periodically, so that the alignment of the LCD does not become fixed. When this AC signal changes, however, electromagnetic noise is generated from the LCD.
With a resistive film type of touch panel, a resistance value changes linearly, in response to the position that is touched. Using this characteristic and the principle of a resistive voltage divider, a voltage is applied to the X axis in order to read the position on the Y axis, the Y axis coordinate being read by the ratio of detected voltage between the Y terminals.
Similarly, a voltage is applied to the Y axis in order to read the position on the X axis, the X axis coordinate being read by the ratio of detected voltage between the X terminals.
This method is used to detect the coordinates of a position that is touched on a touch panel.
To obtain data from a touch panel, an A-D (analog-digital) converter is used. However, because of the high impedance of the A-D converter, the touch panel is relatively susceptible to the influence from electromagnetic noise that intrudes from outside.
Thus, if electromagnetic noise that is generated from the LCD is applied to the touch panel, an error occurs in the detected voltage, thereby preventing accurate acquisition of coordinate data.
In the past, the following known methods existed for avoiding the influence of the above-noted noise. For example, a transparent conductive sheet that is connected to the ground of the touch panel circuit was inserted between the LCD and the touch panel, so as to electrically reduce the noise generated from the LCD.
This method shall be referred to as the first example of prior art. Another method was based on the knowledge that there is a large amount of noise that is generated from the LCD at the transformation point of the AC signal applied for the purpose of changing the alignment of the LCD.
In this method, which will be referred to as the second example of prior art, the transformation point of the AC signal is detected, and the capture of data of the touch panel is stopped at this AC signal transformation point.
An additional method, referred to herein as the third example of prior art, is disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 4-15723.
In this example of prior art, the conductivity an ITO (indium tin oxide) film that is part of the touch panel is made small. That is, the problem of susceptibility to external noise caused by a high impedance is resolved by making the impedance between the ITO films low.
A fourth example of prior art is disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 4-371916.
In this example, to avoid the influence on a touch panel that is formed together with the LCD panel by noise from the LCD panel, the LCD AC signal is captured and a delay circuit is used to delay it by a prescribed amount of time. A single-pulse scan signal is generated at the rising edge of the delay signal that is output by the delay circuit.
Additionally, by capturing data from the touch panel in synchronization with this scan signal, there is a substantial elimination of noise from the touch panel.
In an additional fifth prior art example, which is disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 5-6153, a dummy electrode in proximity to the touch panel is used to pick up noise from the LCD. The difference between this signal from this dummy electrode and the touch panel signal is taken as the actual touch panel signal so as to substantially eliminate the noise on the touch panel.
The above-noted prior art methods, however, have the following problems. In the first prior art example, in order to electrically reduce the LCD noise, a transparent electrically conductive sheet is fitted between the touch panel and the LCD, thereby providing an electrical shield.
With this construction, however, even though the sheet is transparent, inserting the electrically conductive sheet LCD reduces the LCD contrast, and in actual use the LCD display becomes difficult to view, making this method impractical.
In the second prior art example, the capture of data is stopped at the transformation point of the AC signal. In this second prior art example, it is necessary to have a means of stopping midway the operation of the A-D converter that detects the position on the touch panel.
By stopping the operation of the A-D converter midway, an extra time lag occurs, thereby creating a substantial drop in the sampling rate of the A-D converter. As a result, there is a possibility of a loss of performance.
In the third prior art example, a low-resistance material is used for the material of the ITO film, thereby lowering the overall touch panel impedance so as to reduce the influence from external noise.
In the third prior art example, however, because a low-resistance material is used in the ITO film of the touch panel, when a voltage is applied in order to detect a position, a large amount of current flows between the terminals, this resulting in a large power consumption.
In the fourth prior art example, the LCD AC signal is captured and this AC signal is delayed by a prescribed amount of time, the delayed signal being use to trigger the A-D converter. In this case, the touch panel data capture timing is basically dependent upon the LCD AC signal.
If the touch panel data capture timing is dependent upon the LCD AC signal, however, the following problem occurs.
If the frequency of the AC signal is lower than the sampling frequency of the A-D converter that captures the touch panel data, sampling is performed each time the sampling start trigger is generated by the AC signal, this meaning that the number of samples taken is reduced, so that it is not possible capture meaningful data from the touch panel.
Also, even if data for several points are captured each time the sampling start trigger is generated by the AC signal, unless there is synchronization between the sampling rate and the AC signal frequency, non-uniform sampling will result.
In the fifth prior art example, a dummy electrode is used in order to eliminate noise generated by the LCD.
In this fifth prior art example, because of the use of a dummy electrode to eliminate noise on the touch panel, the cost is high compared to the case in which a dummy electrode is not used.
Also, when fitting the dummy electrode between the touch panel and the LCD, there is the problem of a loss of contrast in the LCD, similar to the case of the first prior art example.
Accordingly, it is an object of the present invention to provide a liquid crystal display accompanied by a touch panel directly attached thereto or with a touch panel provided closely adjacent thereto, which is capable of achieving a touch panel sampling rate that is not dependent upon the AC signal frequency, and of eliminating adverse affect to an image display caused by noise generated by the LCD and added to data used in the touch panel.